Element-Specific Characterization of Transient Electronic Structure of Solvated Fe(II) Complexes with Time-Resolved Soft X-ray Absorption Spectroscopy

Polypyridyl transition-metal complexes are an intriguing class of compounds due to the relatively facile chemical designs and variations in ligand-field strengths that allow for spin-state changes and hence electronic configurations in response to external perturbations such as pressure and light. Light-activated spin-conversion complexes have possible applications in a variety of molecular-based devices, and ultrafast excited-state evolution in these complexes is of fundamental interest for understanding of the origins of spin-state conversion in metal complexes. Knowledge of the interplay of structure and valence charge distributions is important to understand which degrees of freedom drive spin-conversion and which respond in a favorable (or unfavorable) manner. To track the response of the constituent components, various types of time-resolved X-ray probe methods have been utilized for a broad range of chemical and biological systems relevant to catalysis, solar energy conversions, and functional molecular devices. In particular, transient soft X-ray spectroscopy of solvated molecules can offer complementary information on the detailed electronic structures and valence charge distributions of photoinduced intermediate species: First-row transition-metal L-edges consist of 2p-3d transitions, which directly probe the unoccupied valence density of states and feature lifetime broadening in the range of 100 meV, making them sensitive spectral probes of metal ligand interactions. In this Account, we present some of our recent progress in employing picosecond and femtosecond soft X-ray pulses from synchrotron sources to investigate element specific valence charge distributions and spin-state evolutions in Fe(II) polypyridyl complexes via core-level transitions. Our results on transient L-edge spectroscopy of Fe(II) complexes clearly show that the reduction in sigma-donation is compensated by significant attenuation of pi-backbonding upon spin-crossover. This underscores the important information contained in transient metal L-edge spectroscopy on changes in the 3d orbitals including oxidation states, orbital symmetries, and covalency, which largely define the chemistry of these complexes. In addition, ligand K-edge spectroscopy reveals the "ligand view" of the valence charge density by probing 1s-2p core-level transitions at the K-edge of light elements such as nitrogen, carbon, and oxygen. In the case of Fe(II) spin-conversion complexes, additional details of the metal ligand interactions can be obtained by this type of X-ray spectroscopy. With new initiatives in and construction of X-ray free-electron laser sources, we expect time-resolved soft X-ray spectroscopy to pave a new way to study electronic and molecular dynamics of functional materials, thereby answering many interesting scientific questions in inorganic chemistry and material science.